The present invention relates to tilt switches which open or close an electric circuit when the switch is subjected to a predetermined angle of tilting.
Such a tilt switch is disclosed, for example, in Bitko U.S. Pat. No. 4,090,040 issued Jan. 16, 1979. A switch disclosed therein comprises a housing forming an internal chamber. One or more electrical terminals extend into the chamber. A mass of electrically conductive liquid, such as mercury, is disposed within the chamber and is movable between positions in and out of contact with the electric terminal(s) in response to tilting of the switch housing.
A tilt switch of that type is capable of providing a signal when an object to which the switch is attached is tilted by a predetermined angle. A shortcoming of such a tilt switch becomes evident in cases where the tilt switch is subjected to shock, such as a momentary impact or prolonged vibration of such magnitude as to cause the mercury to bounce around within the chamber and intermittently open and close the contacts.
To avoid that problem, it has previously been proposed to fill the rest of the chamber with a damping fluid, such as oil. The mercury is denser than the oil and thus still behaves in the above-described manner, except that the oil tends to dampen the movement of the mercury and thus eliminate the ability of the mercury to bounce around in the chamber. However, a shortcoming of that type of switch involves the fact that in response to shock, the mercury tends to fragment into small spheroids, whereby the switch is inoperable until the spheroids coalesce back into a unified mass. In the absence of damping oil, the spheroids would coalesce quickly, but in the presence of the damping oil, the coalescence of the spheroids is retarded and can take a considerable period of time, depending upon the number and size of the spheroids, as well as upon the viscosity of the damping oil.
Therefore, it will be appreciated that mercury tilt switches have not been particularly useful in cases where the switch would be expected to encounter appreciable shock.